This invention relates to electric vehicles and, in particular, to power systems for electric vehicles.
Conventional electric vehicles often include a hybrid power system including both a battery and a fuel cell. See, e.g., U.S. Pat. Nos. 4,839,574; 4,931,947; 4,961,151; 4,962,462; 5,631,532; and 5,808,448. Hybrid power systems are used because conventional batteries lack sufficient charge when used alone to enable travel over long distances and also require relatively long periods of time to charge. Further, conventional fuel cells are unable to generate sufficient power when used alone to accommodate wide variations in the power requirements of the vehicle. Conventional fuel cells also have long start times in cold ambient temperatures and respond slowly to changes in vehicle power requirements.
Most conventional hybrid power systems used in electric vehicles include a relatively large fuel cell that is used as the primary power source for the vehicle and a relatively small battery that is used as a secondary power source when vehicle power requirements exceed the maximum power output of the fuel cell. These conventional hybrid power systems are disadvantageous, however, because fuel cells are relatively expensive and the overall power system still responds slowly to changes in vehicle power requirements.
The present invention provides a hybrid power system for supplying power to a load such as an electric vehicle. A hybrid power system in accordance with the present invention includes an energy storage device, such as a battery, and a fuel cell. The energy storage device supplies all of the power to the load as long as the state of charge of the energy storage device is greater than a first predetermined state of charge. The fuel cell supplies at least a portion of the power to the load when the state of charge of the energy storage device is less than or equal to the first predetermined state of charge.
In accordance with additional aspects of the present invention, the amount of power provided by the energy storage device and the fuel cell may be varied responsive to certain operating conditions to optimize the efficiency of the power as; system. In particular, the fuel cell may supply all of the power to the load when the state of charge of the energy storage device falls below a second predetermined state of charge. When the energy storage device""s state of charge falls between the first and second predetermined states of charge, the fuel cell may supply all of the power to the load as long as the power requirement of the load is less than or equal to an optimal power output of the fuel cell. When the power requirement of the load exceeds the optimal power output of the fuel cell, both the energy storage device and fuel cell may supply power to the load.
A method in accordance with the present invention for supplying power to a load includes the step of providing an energy storage device and a fuel cell. The method further includes the step of controlling the energy storage device and fuel cell wherein the energy storage device supplies all of the power to the load when the state of charge of the energy storage device is greater than a first predetermined state of charge and the fuel cell provides at least a portion of the power when the state of charge of the energy storage device is less than or equal to the first predetermined state of charge.
The present invention represents an improvement as compared to conventional hybrid power systems because the inventive power system does not require a large fuel cell and, therefore, is less expensive than conventional power systems. Further, the inventive power system is able to respond relatively quickly to variations in power requirements by the load. Finally, the inventive power system controls the energy storage device and fuel cell so as to optimize the power generating capabilities of the energy storage device and fuel cell.
These and other advantages of this invention will become apparent to one skilled in the art from the following detailed description and the accompanying drawings illustrating features of this invention by way of example.